(1) Field of the Invention
There are four basic types of electroacoustic transducers in use for underwater sound transmission which will respond to an electrically programmable input signal. The first, is the piezoelectric transducer and the second is the magnetostrictive transducer. However, because of physical properties, these two are impractical to use at low frequencies. The third type is the hydraulic or hydro-acoustic transducer which is capable of low frequency, high power operation. This one is incapable of broadband operation.
The invention falls into the fourth category, electrodynamic or moving coil, piston type of transducer. This type of transducer is characterized by broadband operation, relatively small size, one moving part mechanism, low frequency capability and accurate reproduction of input signal. This type of transducer has been employed mainly as a laboratory or controlled environment calibration device because of its ability to accurately reproduce an input signal. The use of the device as an ocean going sound source has been somewhat limited because of several disadvantages.
(2) Description of the Prior Art
Moving coil, piston transducers rely upon a gas filled chamber which gives compliance to the piston motion. To operate at varying depths it is necessary to pressurize the gas to the external hydrostatic pressure. The change in gas pressure is accomplished by the use of one of several devices designed for that purpose. One disadvantage is the change in performance characteristics with pressure. The change is brought about by the variation of gas density with pressure and associated changes in viscosity and compliance. It is explained below that several facets of the invention minimize the undesirable effects.
The other disadvantage is the demonstrated low reliability of moving coil transducers. There are several areas of transducer operation which have historically been responsible for the high failure rate. These include: pressure compensation system flow rate inadequacy, coil overheating, leadwire fatigue, piston misalignment, shock due to piston hitting displacement stops, coil wire/coil form separation, seal failure and high voltage connector breakdown. The description below will show how several facets of the invention will significantly reduce the failure rate in these particular areas resulting in reliability factor equal to or better than that of alternative devices.